This invention relates generally to power generation systems in general and more specifically geothermal power generation systems.
Power generation systems or power plants are well-known in the art and are widely used to produce useful work (e.g., electricity) from heat sources. Most such power generation systems generate electricity from heat energy derived from burning fossil fuels (e.g., coal or natural gas) and are referred to herein as thermal power plants. In addition to using heat energy derived from burning fossil fuels, thermal power plants can also be used with a wide variety of other heat sources, such as solar, geothermal, and nuclear sources.
Traditionally, thermal power plants have operated in accordance with the well-known Rankine thermodynamic cycle, although other cycles are known and are being used. In the Rankine cycle, a working fluid, such as water, is heated to produce steam. The steam is then expanded, typically through a turbine, in order to convert into mechanical work the heat energy contained in the working fluid. In the case of an electric power generation system, the turbine is operatively connected to an electrical generator which produces the electricity. While power plants operating in accordance with the Rankine cycle typically use water as the working fluid, other types of working fluids are known and may be better suited to the particular type of heat source utilized and the thermodynamic cycle of the system.
For example, in a geothermal power system, useful work (e.g., electricity) is extracted from heat energy contained in geothermal brine extracted from the earth. While geothermal power generation systems have been constructed that utilize the geothermal brine as the working fluid (i.e., in a xe2x80x9cdirect flashxe2x80x9d type of geothermal system), it is sometimes advantageous to utilize a so-called xe2x80x9cbinaryxe2x80x9d system in which the heat from the geothermal brine is transferred to a recirculating working fluid. The recirculating working fluid is then used to drive the energy conversion device (e.g., the turbine). The use of a separate, recirculating working fluid dispenses with the need to design the turbine to operate with the geothermal brine.
In a binary type geothermal power generation system, a vapor generator system (e.g., a heat exchanger) receives the hot geothermal brine and allows the heat energy contained therein to heat and vaporize the recirculating working fluid. The vaporized working fluid is then expanded through the turbine to produce useful work. The working fluid exhaust stream from the turbine is then condensed, e.g., converted back into a liquid. Thereafter, the condensed working fluid is re-circulated to the vapor generator, whereupon it is re-vaporized and again expanded through the turbine.
While the working fluid for such a binary geothermal power system may comprise water, it is generally preferable to use a working fluid that comprises a volatile organic compound (VOC), such as isobutane or isopentane. Such VOC working fluids are generally better suited for use with the pressure and temperature regimes associated with geothermal power generation systems.
However, regardless of the particular type of working fluid that is utilized, one problem associated with power generation systems in general and geothermal power generation systems in particular, relates to the accumulation of so-called non-condensible gases (NCGs) in the working fluid. Such gases are referred to as xe2x80x9cnon-condensiblexe2x80x9d since they do not condense at the temperatures and pressures at which the working fluid is condensed. That is, the condensation of the working fluid in the condenser system generally does not result in the condensation of the NCGs. In a system utilizing a VOC working fluid, such non-condensible gases typically comprise air and can come from a variety of sources, including the turbine lubricant treatment and recycle system, impurities in the working fluid, air introduced during system repairs, as well as from small leaks which may be present in the system.
While the accumulation of NCGs does not pose a serious problem in the high pressure side of the power generation system (e.g., in the vapor generator and turbine systems) in which is used the working fluid, they can cause problems in the low pressure side of the system, particularly in the condenser. More specifically, the non-condensible gases (NCGs) tend to accumulate at the vapor/liquid interface in the condenser, restricting the movement of the vapor stream to the vapor/liquid interface and lowering the partial pressure of the vapor at the vapor/liquid interface. The result is a decreased heat transfer coefficient in the condenser system and a higher condenser pressure, both of which adversely affect the overall efficiency of the system and result in reduced power output.
Standard practice in binary type geothermal power plants is to allow the NCGs to accumulate until an unacceptable level is reached. The working fluid vapor is vented and the NCGs removed in a batch process. The purified working fluid is then returned to the system. While this practice is effective from a functional standpoint, it is less than ideal in that the efficiency of the power generation system is continuously reduced until such time as the accumulated NCGs are removed from the system. Then, as the NCGs again accumulate, the efficiency of the system is again gradually reduced until the NCGs are again removed or purged from the system. Depending on the specific power generation system, the gradual accumulation and periodic purging of NCGs can reduce annual production by as much as 2 to 4 percent.
Apparatus for removing non-condensible gas from a working fluid utilized in a thermodynamic system may comprise a membrane having an upstream side operatively connected to the thermodynamic system so that the upstream side of the membrane receives a portion of the working fluid. The first membrane separates the non-condensible gas from the working fluid. A pump operatively associated with the membrane causes the portion of the working fluid to contact the membrane.
Also disclosed is a method for removing non-condensible gases from a working fluid utilized in a thermodynamic system that comprises continually separating non-condensible gas from the working fluid during operation of the thermodynamic system. The step of continually separating the non-condensible gas from the working fluid may comprise the steps of contacting with a membrane a portion of the working fluid in a vapor state, the membrane separating the non-condensible gas from the working fluid in the vapor state; and returning to the thermodynamic system the working fluid separated from the non-condensible gas.